The present invention relates to the magnetic resonance imaging arts. It finds particular application in conjunction with the separation of water and fat signals from human patients and will be described with particular reference thereto. However, it is to be appreciated that the present technique is also applicable to imaging sequences in which components of the imaged region have very close resonance frequencies.
Heretofore, subjects have been positioned in a temporally constant magnetic field such that selected dipoles preferentially align with the magnetic field. Radio frequency signals have been applied to cause magnetic resonance of the preferentially aligned dipoles. The radio frequency magnetic resonance signal from the resonating dipoles has been read out for reconstruction into an image representation.
To strengthen the magnetic resonance signal, the resonance can be excited with a 90.degree. radio frequency pulse followed by a 180.degree. refocusing pulse. The 180.degree. refocusing pulse causes the resonating spin system to refocus as a spin echo. The time between the 180.degree. refocusing pulse and the spin echo is the same as the time between the 90.degree. excitation pulse and the 180.degree. refocusing pulse.
Magnetic resonance echoes can also be induced by other disturbances of the spin system, such as reversing the polarity of a magnetic field gradient to induce a gradient echo.
As illustrated in U.S. Pat. No. 4,833,407 of Holland, Provost, DeMeester, and Denison, spin and gradient echoes have been induced interleaved in the same repetition of the magnetic resonance imaging sequence. Briefly summarized, an RF excitation and refocusing pulse were applied to induce a spin echo. The polarity of a magnetic field gradient along a read axis was reversed one or more times to induce one or more gradient echoes. When used with fast spin echo (FSE) techniques, refocusing radio frequency pulses are applied after each spin echo to induce yet another spin echo.
As indicated above, water and fat have close resonance frequencies, but differ by about 220 Hertz in a magnetic field of 1.5 Tesla. In the 1.5 Tesla field, the 220 Hertz higher frequency component gains a full revolution on the slower component every 4.46 msec. That is, the signals are in-phase with a 4.46 msec. periodicity. By inducing one echo at 2.23 msec. after refocusing and another echo at 4.46 msec. after refocusing, a pair of echoes can be induced and their signals read out. One of the echo signals has the water and fat in-phase and the other has the water and fat signals 180.degree. out-of-phase. The two echoes can be induced in two different repetitions of the spin echo imaging sequence by shifting the radio frequency refocusing pulse by 2.23 msec. See, for example, Glover, et al., "Three-Point Dixon Technique For True Water/Fat Decomposition with B.sub.0 Inhomogeneity Correction", Magnetic Resonance in Medicine, Vol. 18, pp. 371-383 (1991). One of the problems with this technique is that the system is typically not sufficiently linear that one can add the in-phase and out-of-phase signals to get a water only signal and subtract the in-phase and out-of-phase signals to obtain a fat only signal. The time evolution of the fat and water signals is sufficiently non-linear that in-phase and out-of-phase components do not cancel completely. To correct for this non-linearity, the Glover, et al. technique generates three echoes--one at a nominal echo time, one 2.23 msec. advanced from the nominal echo time, and one 2.23 msec. retarded from the nominal echo time. Through the use of post-processing, Glover seeks to remove the non-linearities such that components of 2.23 msec. displaced echoes substantially cancel. Another drawback of the Glover technique is that three repetitions of the imaging sequence are required to generate magnetic resonance echoes with retarded, advanced, and reference timings.
The present invention contemplates a new and improved imaging technique which overcomes the above-referenced problems and others.